The present invention relates to an electrophoretic display device and a process for production thereof.
Along with development of data processing apparatus and devices in recent years, there is an increasing demand for so-called xe2x80x9cpaper-like displayxe2x80x9d which is rewritable electronic paper satisfying the convenience of paper medium and electronic medium function for a portable personal computer. A display device used for such a paper-like display should require a small power consumption, be thin and have a flexibility like paper durable against flexure. One sheet or plural sheets of such display devices are used as a paper-like display.
Display devices in compliance with such a demand have been a subject of extensive research and development. Among these, a liquid crystal display device wherein an alignment of liquid crystal molecules is electrically controlled to change optical characteristics has been extensively developed and commercialized as a display device satisfying the demand described above. However, such liquid crystal display devices are still accompanied with problems of visual load on human eyes, such as difficulty of recognizing characters on display depending on a viewing angle or due to reflection light, and flickering and low luminance of light source. Further, because of necessity of alignment control of liquid crystal molecules and accurate control of liquid crystal cell gap, it is difficult to also satisfy a flexibility of the device.
As a type of reflection display device capable of realizing a flexibility, there has been known an electrophoretic display device wherein colored charged particles are moved within an insulating liquid to effect a display (e.g., as disclosed in U.S. Pat. No. 3,668,106). FIGS. 4A and 4B are schematic sectional views for illustrating an organization and an operating principle of a representative one of such an electrophoretic display device.
Referring to FIGS. 4A and 4B, an electrophoretic display device includes an electrophoretic dispersion liquid comprising an insulating liquid (colored dispersion medium) 47 and colored charged particles (electrophoretic particles) 46 dispersed therein, and a pair of oppositely disposed pair of electrodes 42 and 43 sandwiching the electrophoretic dispersion liquid. By applying a voltage across the electrophoretic display device via the electrodes 42 and 43, the electrophoretic particles 46 are electrophoretically moved to and fixed on either one of the electrodes 42 and 43 which are biased to mutually opposite polarities to effect a display. The display is effected based on the color of the electrophoretic particles 46 and the dyed color of the colored dispersion medium 47. More specifically, in a state shown in FIG. 4A wherein the electrophoretic particles 46 are attached to a first electrode 42 closer to a viewer, the color of the particles 46 is displayed. On the other hand, in a state shown in FIG. 4B wherein the particles are moved and attached to a second electrode 43 more remote from the viewer, the color of the dispersion medium 47 is displayed.
The display device can be formed in a thin shape In principle and formed in a device retaining some degree of flexibility by using flexible materials for the substrates and other associated members including electrodes.
Further, a display picture retention characteristic (or a display memory characteristic) is provided by making the circuit open immediately after the voltage application to hold some charge at the electrodes exerting a Coulomb force for attracting or adsorbing the colored electrophoretic particles.
By suppressing the reduction in charges held by the electrodes, e.g., by disposing an insulating layer of an appropriately selected material, the electrophoretic particles can be retained on the electrodes for a relatively long period, so that it is possible to continually retain a display memory characteristic for long hours without supplying any external power.
An electrophoretic display deice operates at a relatively low current compared with other types of display devices, such as a liquid crystal device, in principle, and can further reduce the average power consumption in a case where a frequent rewriting of display is not required.
However, a conventional electrophoretic display device is accompanied with the following problems.
A first problem is encountered at the time of injecting the electrophoretic dispersion liquid between the substrates in the process for producing the electrophoretic display device. In an example of the production process, two substrates for constituting the electrophoretic device is applied to each other while leaving a port for injecting the electrophoretic dispersion liquid. Thereafter, the electrophoretic dispersion liquid is injected through the injection port, but the smooth injection thereof is obstructed by the presence of a spacer and partitioning walls for keeping the spacing between the substrates at a constant. Further, in the case of injecting the electrophoretic dispersion liquid from an edge of the device toward the edge on the opposite side, the dispersion medium can be easily transferred but the electrophoretic particles are liable to be caught at the spacer and partitioning walls, so that the electrophoretic particle concentration is liable to be high near the injection port and lowered at points leaving away from the injection port, thus adversely affecting the display uniformity.
In another example of the production process, a lower substrate is first provided with spacer and partitioning walls, an electrophoretic dispersion liquid is uniformly distributed thereover, and thereafter an upper substrate is bonded to the lower substrate to seal the periphery of the structure. According to this process, a uniform concentration of electrophoretic particles over the planar extension of the device can be realized, but a problem of entrainment of air bubbles is liable to occur at the time of bonding the upper substrate.
A second problem is encountered as a difficulty of realizing a device showing paper-like flexibility. This is because an electrophoretic. display device is composed of two substrates so that it is difficult to absorb a difference between an inner circumference and an outer circumference caused when the device is flexurally deformed even if the two substrates are composed of flexible materials. Thus, the flexured device is liable to result in a stiff touch.
A third problem is liable to be encountered when a flexible device is flexurally deformed. Thus, at the time of deformation, the insulating liquid therein is flowed to push the electrophoretic particles adsorbed by electrostatic interaction with the electrode retention charge, so that the display is liable to be deformed to remove the display memory characteristic.
A generic object of the present invention is to provide an electrophoretic display device and a process for production thereof having solved the above-mentioned problems of the prior art.
A more specific object of the present invention is to provide an electrophoretic display device having a flexibility susceptible of easily following flexural deformation.
Another object of the present invention is to provide an electrophoretic display device free from loss of display memory characteristic even when subjected to deformation.
A further object of the present invention is to provide a process for producing an electrophoretic display device allowing easy injection and sealing of an electrophoretic dispersion liquid.
According to the present invention, there is provided an electrophoretic display device for effecting a display by moving electrophoretic particles, comprising: a flexible support sheet, a plurality of partitioning walls formed on the support sheet, a ceiling sheet disposed over the partitioning walls and opposite to the support sheet so as to form a plurality of cells defined by the support sheet, the ceiling sheet and the partitioning walls, and a electrophoretic dispersion liquid filling the cells and comprising an insulating liquid and electrophoretic particles dispersed in the insulating liquid, wherein the ceiling sheet is flexible and also capable of expansion and contraction upon application of a planar stress.
According to another aspect of the present invention, there is provided a process for producing an electrophoretic display device for effecting a display by moving electrophoretic particles dispersed in an insulating liquid while changing a direction of electric field acting on the electrophoretic particles, comprising the steps of:
disposing a plurality of partitioning walls on a flexible support sheet to form a plurality of sections defined by the support sheet and the partitioning walls,
injecting an electrophoretic dispersion liquid comprising an insulating liquid and electrophoretic particles dispersed therein into the sections on the support sheet,
applying a hardenable liquid immiscible with the insulating liquid over the partitioning walls and the electrophoretic dispersion liquid therein, and
hardening the hardenable liquid to form a ceiling sheet thereby defining a plurality cells within which the electrophoretic dispersion liquid is sealed up.
Thus, by using a ceiling sheet capable of expansion and contraction upon application of a planar stress, i.e., a stress acting in a plane extension thereof, the electrophoretic display device of the present invention becomes easily pliable in response to flexural deformation. Further, a flexural deformations stress acting on the device can be absorbed by deformation of the ceiling sheet, so that the flow of the electrophoretic display device in the device is suppressed to provide the electrophoretic display device with a better display memory characteristic.
Further, according to the process of the present invention, it is possible to produce an electrophoretic display device, while easily achieving uniform distribution and sealing into respective display sections of the electrophoretic dispersion liquid.